1. Field of the Invention
This invention relates to sag bending of glass sheets on bending molds and in particular to a method and apparatus for press bending selected portions of the glass sheets on bending molds while moving through a heating lehr.
2. Technical Considerations
In the practice of sag bending to form shaped glass windows for automobiles and the like as disclosed in U.S. Pat. No. 4,375,978 to Reese, glass sheets are positioned on and supported by a skeletal bending mold. The shaping rail of the mold has a shape and configuration similar to that of the shaped glass sheet at a location slightly inboard of its peripheral edge. The bending molds are then conveyed in succession through a heating lehr where the glass sheet is heated to its deformation temperature such that it begins to sag by gravity until the glass sheet conforms to the configuration of the shaping rail. After the glass sheet is shaped, the mold is conveyed through an annealing zone where the glass sheet is cooled in a controlled manner from its deformation temperature through its annealing range to anneal the glass sheet. The glass sagging technique has been the method used to bend two glass sheets, or doublets, simultaneously which sheets are subsequently laminated together to form a laminated automobile windshield. The windshield is curved to conform and blend into the shape of an automobile vehicle in which it is installed.
A critical shape parameter of curved glass sheets used for windshields is the approach angle of the glass sheets along the A post of the vehicle body. The approach angle is the angle at which the windshield meets the vehicle body at the generally vertically extending A-posts of the window frame. It has been found that in sag bending glass sheets with deep sag or reverse curvatures, there is a tendency for the sheets to draw glass from their longitudinal end sections. As a result, the glass sheets may tend to lift off the shaping rail of the outline bending mold and have reduced curvature causing the sheets to deviate from the desired shape and tolerances. This deviation may be caused by overheating the glass sheet along the outer edge of its longitudinal sections to achieve the desired curved configuration.
As automotive stylists strive for more aerodynamic designs, the windshields are assuming more complex and deeper bend configurations. In addition, the windshield edges are approaching the A-post of the vehicle at a more flush fashion to provide a smoother transition between the windshield surface and the vehicle body surface. As the windshield designs become more complicated with compound and complex curvatures, these shapes are becoming increasingly more difficult to control during conventional sag bending operations.
It would be advantageous to develop a method of forming glass sheets and incorporating conventional sag bending techniques with other shaping techniques so as to form and maintain the desired curvatures required for proper vehicle assembly.